Technical Abstract:
The goal of this research is to utilize current advances in Rosaceae genomics to identify DNA markers for use in marker-assisted selection of durable resistance to fire blight. Candidate fire blight resistance genes were selected and ranked based upon differential expression after inoculation with Erwinia amylovora, bioinformatic analysis of sequence information and inference from the scientific literature. Single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers derived from highly ranked ESTs (candidate genes) were mapped in bi-parental families segregating for fire blight resistance. Candidate gene markers that co-localized with known QTLs for fire blight resistance were then selected for further analysis and marker evaluation. This approach was successful in identifying two markers derived from a leucine rich repeat receptor-like protein (LRR-RLP) and a class III peroxidase (Prx) that were associated with the fire blight resistance QTL on linkage group 3 of Malus ‘Robusta 5’ (New Zealand accession). The LRR-RLP is predicted to localize to the outside of the cell and contains a conserved 24 amino acid consensus motif associated with extracytoplasmic plant resistance gene LRRs. Prx genes are ascribed a wide variety of functional roles including defense against bacterial pathogens. This approach is now being applied to other sources of fire blight resistance in M. sieversii. The results demonstrate the utility of combining functional genomics and a candidate gene approach to identify molecular markers for complex biological traits.